1. Field of the Invention
The present invention relates to a separator for a non-aqueous rechargeable lithium battery.
2. Description of the Related Art
Conventional non-aqueous rechargeable lithium batteries have a separator including an electrically insulating porous film between a positive electrode and a negative electrode, and an electrolyte solution in which dissolved lithium salt is impregnated in the film pores. Such a non-aqueous rechargeable lithium battery has excellent characteristics such as high capacity and high energy density.
The positive electrode or the negative electrode of conventional non-aqueous rechargeable lithium batteries has an active material of a transition metal compound. For example, when the positive active material includes a Li—Co-based oxide, such as LiCoO2, trivalent cobalt ions (Co3+) are converted to an unstable quadrivalent cobalt ions (Co4+) by the intercalation reactions of lithium ions during the charge and discharge cycles. The quadrivalent cobalt ions then react with an electrolyte solution to oxidize the electrolyte solution and are reduced to divalent cobalt ion (Co2+). The divalent cobalt ions tend to dissolve in the electrolyte solution, and move to the negative electrode, where they are reduced and precipitated as cobalt metal. When the cobalt metal is deposited and accumulated in the negative electrode, the separator and/or the negative electrode may get damaged and the charge and discharge characteristics of the non-aqueous rechargeable lithium battery may also deteriorate accordingly.
In addition, when the quadrivalent cobalt ions are reduced to divalent cobalt ions, the separator may also oxidize and deteriorate. For example, when a separator is composed of polyethylene, the hydrogen atoms are removed by the oxidation reaction, leaving behind only carbons as backbones in the polyethylene molecules. Such a carbonated separator can easily be damaged because of its weak mechanical strength.
As mentioned above, conventional non-aqueous rechargeable lithium batteries have charge and discharge problems because the electrolyte solution decomposition material is deposited on the negative electrode (or positive electrode), and because of the oxidized separator.
Furthermore, due to the high capacity and high energy density in the conventional rechargeable battery, short circuits could occur both inside and outside of the battery, and the battery temperature could rapidly increase. For these reasons, conventional separators typically include a porous film of polyethylene having a melting point ranging from 120 to 140° C. These separators have excellent shutdown characteristic and handling property, and are low in cost.
The shutdown characteristic refers to the core being closed and the current being blocked due to a battery temperature increase from an overcharge or an inside or outside short, so a part of the separator is fused. In other words, as the battery temperature increases, the separator fuses and rapidly contracts, or gets damaged, so the short circuit occurs.
Therefore, in order to improve the safety of the non-aqueous rechargeable lithium battery, it has been attempted to improve the heat resistance of the electrode materials, particularly the separators. Various attempts to improve or ensure battery safety even when the separator is rapidly contracted or damaged have been tried.
Other attempts to improve or ensure battery safety include providing a rechargeable battery mounted with a separator including a heat-resisting nitrogen aromatic polymer and a ceramic powder; a rechargeable battery mounted with a porous film composed of an inorganic oxide filler and a film binder on the surface of a positive electrode or a negative electrode; and a rechargeable battery mounted with a separator including a mixture of a filler having a melting point of 250° C. or more and a filler having a melting point ranging from 80 to 120° C.
However, in such rechargeable batteries, the deterioration of a rechargeable battery due to the transition metal included in the electrode has never been considered.